Molar Mass Calculations

Posted on | by Linus

• PhET Concentration https://phet.colorado.edu/en/simulations/concentration

 

Activity 1: Molar Mass Calculations

For this activity, you will need the periodic table in Appendix A of the prelab.

 

Using the periodic table, determine the molar mass of the following compounds:

1. Cobalt(II) nitrate, Co(NO3)2

 

 

 

2. Cobalt(II) chloride, CoCl2

 

 

 

3. Potassium dichromate, K2Cr2O7

 

 

 

4. Potassium chromate, K2CrO4

 

 

 

5. Nickel(II) chloride, NiCl2

 

 

 

6. Copper(II) sulfate, CuSO4

 

 

 

7. Potassium permanganate, KMnO4

 

 

 

8. Sodium chloride, NaCl

 

 

 

 

 

Concentration

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Activity 2: Saturated Solutions

For this activity, you will be using the Concentration simulation on the PhET website. The web address

for this simulation is given at the beginning of the lab handout under Web Links. You can also click here

to take you to the simulation. You are going to investigate the effect of varying the amount of solute,

solvent, and solution on the saturation of the solution.

 

1. Open the PhET Concentration Simulation.

2. Move the crosshairs end of the concentration meter into the solution. It will read 0.000 mol/L. Leave

the solute selection dropdown menu set to “Drink Mix”.Shake the solute shaker a few times. This will

add drink mix to the water. Notice the change in the concentration. In table 1, in the “Drink mix is

added” describe what happens to the concentration, e.g. remains the same, increases, decreases, goes

to zero.

3. Then add additional water to the solution. In table 1 column 3, describe what happens to the

concentration, e.g. remains the same, increases, decreases, goes to zero.

4. Adjust the evaporation slider, so that it is no longer set to “none”. Describe the effect on the

concentration of the solution in table 1 column 4.

5. Drain some (not all) of the solution and describe the effect on the concentration of the solution in table

1 column 5.

6. Click on the “Remove Solute” button and describe the effect on the concentration of the solution in

table 1 column 6.

Figure 1: PhET Concentration Simulation. The arrows point to the pieces that move and affect the concentration and or

amount of solution. The solute can be changed using the drop-down menu at the top right.

 

 

 

 

 

 

 

 

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Table 1: Effect of Various Actions on the Concentration of a Solution

Action

Drink mix is

added Water is added

Evaporation

occurs Solution is drained

Solute is removed

What happens to

concentration?

 

7. Reset the simulation and insert the concentration meter into the solution.

8. Keep the solute to “Drink mix” and add as much as you can.

9. How do you know when a solution is saturated?

 

 

10. While the solution is saturated, add additional solid solute. What happens?

 

 

11. Why do you think this is?

 

 

12. How does adding this additional solute change the concentration of the saturated solution?

 

 

13. Increase the evaporation of the saturated solution. How does evaporation change the concentration of

the saturated solution?

 

 

 

 

 

 

Concentration

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Activity 3: Solute Mass of Saturated Solutions

For this activity, you will continue using the Concentration simulation on the PhET website. You are

going to use the simulation to determine the saturation concentration of different solutes dissolved in

water. Then you will calculate the mass of the solute in the saturated solutions.

 

1. Reset the Concentration Simulation.

2. Fill the beaker to 1 L.

3. Keep the evaporation set to “none”, and place the concentration sensor in the solution.

4. For each solute in table 2, find and record the concentration when saturation occurs. (You can do this

by shaking the solute shaker until the word “saturated” appears. Then read the concentration on the

sensor.) Complete the second column of table 2 with your results.

Table 2: Concentration and Solute Mass for Various Solutes

Solute Concentration for

Saturation

(mol/L)

Solute Mass

(grams)

Drink mix

Cobalt (II) nitrate

Cobalt (II) chloride

Potassium dichromate

Potassium chromate

Nickel (II) chloride

Copper (II) sulfate

Potassium permanganate

Sodium chloride

 

5. Does each solute have the same concentration at saturation?

 

 

6. For each solute, determine the mass in grams required to saturate 1 L of solution. Record these

values in column 3 of the table 2. See example 6 in the prelab for a reminder of how to determine the

mass of a one liter saturated solution.

 

References

1. This lab was adapted from an activity provided by: University of Colorado. (). PhET Interactive

Simulations – Physics, PhET: Interactive simulations. Retrieved at May 7, 2020, from the website

 

SOLUTION

Activity 1: Molar Mass Calculations

To calculate the molar mass of each compound:

  1. Use the periodic table to find the atomic mass of each element.

  2. Multiply the atomic mass by the number of atoms in the formula.

  3. Add the totals for all elements in the compound.

Example Calculations

  1. Cobalt(II) nitrate, Co(NO₃)₂

    • Co = 58.93 g/mol

    • N = 14.01 g/mol × 2 = 28.02 g/mol

    • O = 16.00 g/mol × 6 = 96.00 g/mol

    • Molar Mass = 58.93 + 28.02 + 96.00 = 182.95 g/mol

  2. Cobalt(II) chloride, CoCl₂

    • Co = 58.93 g/mol

    • Cl = 35.45 g/mol × 2 = 70.90 g/mol

    • Molar Mass = 58.93 + 70.90 = 129.83 g/mol

  3. Potassium dichromate, K₂Cr₂O₇

    • K = 39.10 × 2 = 78.20 g/mol

    • Cr = 52.00 × 2 = 104.00 g/mol

    • O = 16.00 × 7 = 112.00 g/mol

    • Molar Mass = 78.20 + 104.00 + 112.00 = 294.20 g/mol

  4. Potassium chromate, K₂CrO₄

    • K = 39.10 × 2 = 78.20 g/mol

    • Cr = 52.00 × 1 = 52.00 g/mol

    • O = 16.00 × 4 = 64.00 g/mol

    • Molar Mass = 78.20 + 52.00 + 64.00 = 194.20 g/mol

  5. Nickel(II) chloride, NiCl₂ = 58.69 + (35.45 × 2) = 129.59 g/mol

  6. Copper(II) sulfate, CuSO₄ = 63.55 + 32.07 + (16 × 4) = 159.55 g/mol

  7. Potassium permanganate, KMnO₄ = 39.10 + 54.94 + (16 × 4) = 158.04 g/mol

  8. Sodium chloride, NaCl = 22.99 + 35.45 = 58.44 g/mol

Tip: Keep a clear table in your lab notebook to track calculations.

Activity 2: Saturated Solutions (PhET Simulation)

Use the PhET Concentration Simulation: https://phet.colorado.edu/en/simulations/concentration

Steps

  1. Place the concentration meter in the solution; read 0.000 mol/L.

  2. Add Drink Mix: concentration increases.

  3. Add Water: concentration decreases.

  4. Increase Evaporation: concentration increases.

  5. Drain some solution: concentration remains the same (concentration depends on ratio of solute to solvent).

  6. Remove Solute: concentration decreases to zero if all solute removed.

Saturation Observations

  • A solution is saturated when additional solute does not dissolve and remains at the bottom.

  • Adding more solid solute does not increase concentration; the solution has reached its maximum solubility.

  • Evaporation of a saturated solution increases concentration because water decreases while dissolved solute remains constant.

Activity 3: Solute Mass of Saturated Solutions

  1. Reset simulation, fill 1 L of water.

  2. For each solute, shake until the word “saturated” appears.

  3. Record the concentration in mol/L.

Calculating Mass of Solute

Use the formula:

Mass (g)=Molarity (mol/L)×Molar Mass (g/mol)×Volume (L)\text{Mass (g)} = \text{Molarity (mol/L)} \times \text{Molar Mass (g/mol)} \times \text{Volume (L)}

Example for Cobalt(II) nitrate:

  • If saturated concentration = 1.0 mol/L

  • Molar mass = 182.95 g/mol

  • Volume = 1 L

Mass=1.0×182.95×1=182.95 g\text{Mass} = 1.0 \times 182.95 \times 1 = 182.95 \text{ g}

  • Repeat for each solute and record in Table 2.

  • Note: Each solute has a different saturation concentration due to solubility differences.

Table 1 Example (Activity 2)

Action Drink Mix is Added Water is Added Evaporation Occurs Solution is Drained Solute is Removed
Effect on Concentration Increases Decreases Increases Remains Same Decreases

Table 2 Template (Activity 3)

Solute Concentration at Saturation (mol/L) Solute Mass (g)
Drink Mix ___ ___
Cobalt(II) nitrate ___ ___
Cobalt(II) chloride ___ ___
Potassium dichromate ___ ___
Potassium chromate ___ ___
Nickel(II) chloride ___ ___
Copper(II) sulfate ___ ___
Potassium permanganate ___ ___
Sodium chloride ___ ___

Key Points to Include in Your Lab Report

  1. Solutes have different saturation concentrations.

  2. Adding solute beyond saturation does not change concentration.

  3. Evaporation increases the concentration of dissolved solute.

  4. Calculating solute mass requires molarity, molar mass, and volume.

If you want, I can fill in Table 2 with estimated saturation concentrations and solute masses for all compounds using standard solubility data so your lab report is mostly complete.

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